This invention relates to heating relatively long intervals of subterranean earth formations at relatively high temperatures for relatively long times. More particularly, it relates to an electrical resistance process of heating which is capable of subjecting an interval of more than several hundred feet of subterranean earth formation to a selected temperature of from about 600.degree. to 1000.degree. C. for a time of more than several years while injecting heat at a rate of more than about 100 watts/foot.
It is known to be beneficial to heat intervals of subterranean earth formations at relatively high temperatures for relatively long times. The benefits obtained may include the pyrolyzing of oil shale formations, the consolidating of unconsolidated reservoir formations, the formation of large electrically conductive carbonized zones capable of operating as electrodes within reservoir formations, the thermal displacement of hydrocarbons derived from oils or tars into production locations, etc. Prior processes for accomplishing such results are contained in patents such as the following, all of which are U.S. patents. U.S. Pat. No. 2,732,195 describes heating intervals of 20 to 30 meters within subterranean oil shales to temperatures of 500.degree. to 1000.degree. C. with electrical heaters having iron or chromium alloy resistors. U.S. Pat. No. 2,781,851 by G. A. Smith describes using a mineral-insulated and copper-sheathed low resistance heater cable containing three copper conductors at temperatures up to 250.degree. C. for preventing hydrate formation, during gas production, with the heater being mechanically supported by steel bands and surrounded by an oil bath for preventing corrosion. U.S. Pat. No. 3,104,705 describes consolidating reservoir sands by heating residual hydrocarbons within them until the hydrocarbons solidify, with "any heater capable of generating sufficient heat" and indicates that an unspecified type of an electrical heater was operated for 25 hours at 1570.degree. F. U.S. Pat. No. 3,131,763 describes an electrical heater for initiating an underground combustion reaction within a reservoir and describes a heater with resistance wire helixes threaded through insulators and arranged for heating fluids, such as air, being injected into a reservoir. U.S. Pat. No. 4,415,034 describes a process for forming a coked-zone electrode in an oil-containing reservoir formation by heating fluids in an uncased borehole at a temperature of up to 1500.degree. F. for as long as 12 months.
In general, as far as the applicants have been able to ascertain, it appears that prior disclosures of methods or devices for heating underground formations at temperatures as high as 600 to 1000.degree. C. for times as long as even one year, have been limited to heating intervals of only a few hundred feet or less and have usually been operated in contact with, and thus cooled by, fluid flowing into or out of reservoir formations. In various situations it can be advantageous to maintain a temperature of about 600.degree. to 1000.degree. C. along an earth formation interval of more than several hundred feet into which heat is injected at a rate of more than about 100 watts/foot for a time longer than several years. However, in the latter type of operation most insulating materials soon become ineffective, most metals used for electrical resistances would require cross-sectional areas which are unfeasibly large or costly, and/or voltages which are unfeasibly high and dangerous. In addition, at those temperatures, metals commonly used for electrical conductors, power supplies, splicing materials or cable sheaths soften and begin to creep or melt.